Infants with cystic fibrosis fed formula containing soy but not cow's milk protein may develop protein malnutrition as their earliest clinical manifestation. Although not well studied, it appears that these infants absorb soy protein poorly. It is possible that the character and content of proteins in the diet have a regulatory effect on protein absorption in pancreatic insufficiency, perhaps through influencing the level of activity of the mucosal phase of digestion. Protein content of the diet is known to influence intestinal peptide hydrolase activity in normal animals. Therefore, this research proposal is designed to evaluate the effect of quantitative and qualitative changes in dietary protein on the mucosal conponent of protein digestion in an animal model of pancreatic insufficiency. In preliminary experiments mucosal peptide hydrolase activities (brush border and cytosol) will be evaluated in the jejunum, mid bowel, and ileum of adult rats following a change from casein hydrolasate to casein or soy protein diet. The time period for maximal change in activity will be determined, as well, for those animals placed on casein diet. All diets will be isocaloric and equivalent in nitrogen content. In subsequent studies, pancreatic secretions will be removed from the small bowel lumen in rats by operative transposition of the common pancreaticobiliary duct. Animals will be fed elemental casein hydrolysate formula during a recovery period of three weeks. Animals with adequate weight gain will be pair fed diet containing: 1) varying concentrations of casein, 2) casein with or without pancreatic enzyme replacement or 3) two qualitatively different proteins (casein or soy). Mucosal peptide hydrolase activity will be determined as in the preliminary studies and expressed per segment, mg DNA, and gm protein for comparison among the diet groups in each of the three studies. Using a standardized radiolabeled protein meal, protein digestion will be compared among animals with pancreatic insufficiency previously pair fed casein and and either casein hydrolysate or soy protein diet. Brush border peptide hydrolases will be purified to raise monospecific antibodies in rabbits for immunochemical studies of enzyme turnover and specific activity of purified enzyme. These studies will serve to evaluate mechanisms of alteration of enzyme activity in the previous experiments.